Now that all the DocBook documentation is gone, we can switch to the
new NixOS documentation generator. No meaningful change in HTML output,
except that I removed the rather pointless preface and changed the
title accordingly. Manual page output is greatly improved; it was
kind of broken before. The `sed` hack is pretty gross but I have
confirmed that nixpkgs will be happy to accept a PR to make things
a little more customizable.
This also drops the `manual` alias (deprecated in nixpkgs in 2018
and imported into nix-darwin), and `manualEpub` (because the NixOS
documentation generator doesn't support it and also nobody wants this
as an ebook).
